Communication apparatus and retransmission control method

ABSTRACT

Provided is a wireless communication device. A PHICH reception unit determines whether a received signal in a PHICH region is an ACK signal or a NACK signal. When doing so, the PHICH reception unit does not receive a PHICH in a subframe in which a terminal monitors an E-PDCCH. A control signal reception unit outputs a retransmission prompting signal to a signal allocation unit when the signal outputted from the PHICH reception unit is a NACK signal and when a UL grant was not detected. Meanwhile, when a UL grant was detected, the control signal reception unit outputs the detected UL grant to the signal allocation unit. The signal allocation unit maps the transmission signal in accordance with the retransmission prompting signal and the UL grant and transmits the transmission signal from a wireless transmission unit.

BACKGROUND

1. Technical Field

The present invention relates to a communication apparatus performing aretransmission control and also to a retransmission control method.

2. Description of the Related Art

In recent years, it has been become common to transmit not only audiodata but also large-volume data such as still-image data and video datain cellular mobile communication systems in connection with theincreasing development of multimedia information. Moreover, inLTE-Advanced (hereinafter referred to as “LTE-A”), techniques enabling ahigh transmission rate using a broad radio band, a MIMO (Multiple-InputMultiple-Output) transmission technique, and an interference controltechnique have actively been discussed in order to enable transmissionof large-volume data.

Furthermore, various apparatuses are introduced as radio communicationterminals in, for example, M2M (Machine to Machine) communication, orthe multiplex number of a terminal increases with a MIMO technique,thereby causing anxiety about a shortage of resources in a region usedfor a control signal (PDCCH: Physical Downlink Control Channel). If acontrol signal cannot be mapped due to this shortage of resources, datacannot be assigned to the terminal. As a result, even if a data regionused as a resource region for mapping data is available, the area cannotbe used, so that a concern arises that the system throughput maydecrease.

As a way of solving this problem, mapping a control signal toward aterminal served the base station to a data region has been discussed.The data region to which the base station maps control signals towardterminals served by the base station are called, for example, an E-PDCCH(Enhanced PDCCH) region, N-PDCCH (New-PDCCH) region, or X-PDCCH region.

Thus, mapping a control signal to a data region enables controlling thetransmission power for a control signal to be transmitted to a terminallocated near a cell edge, the interference given to other cells by acontrol signal to be transmitted or interference given to the cellprovided by the base station from another cell.

However, PHICH (Physical HARQ Indicator Channel) used for uplink (UL)retransmission control transmitted in downlink (DL) indicates anACK/NACK, but the number of bits used herein is small, so thattransmission using a data region results in inefficiency.

Next, a UL retransmission control method of LTE will be described. InLTE, a DL grant (referred to also as DL assignment) serving as DLassignment control information indicating DL data assignment, and a ULgrant serving as UL assignment control information indicating UL dataassignment are transmitted on a PDCCH. DL assignment is used to indicatethat a resource in the subframe used for transmitting this DL assignmenthas been allocated to the terminal.

On the other hand, regarding a UL grant, in a FDD (Frequency DivisionDuplex) system, a UL grant is used to indicate that a resource in thetarget subframe which is the fourth subframe from the subframe used fortransmitting this UL grant has been allocated to the terminal.

Moreover, in a TDD (Time Division Duplex) system, a UL grant is used toindicate that a resource in the target subframe which is the fourthsubframe or a subframe after the fourth subframe from the subframe usedfor transmitting this UL grant has been allocated to the terminal. Inthe TDD system, after how many subframes from the subframe used fortransmitting the UL grant, a subframe is allocated to the terminal as aUL subframe is determined according to a pattern of time-dividing UL andDL (hereinafter, “UL/DL configuration pattern”). However, in any UL/DLconfiguration pattern, the UL subframe is the fourth subframe or asubframe after the fourth subframe from the subframe used fortransmitting the UL grant.

The UL retransmission control supports non-adaptive retransmission forassigning a retransmission signal to the same resource as the resourceallocated last time and adaptive retransmission capable of assigning aretransmission signal to a resource different from the resource to whichthe signal is assigned last time (for example, refer to Non-PatentLiterature (hereinafter, abbreviated as NPL) 1). In the non-adaptiveretransmission, only a PHICH for transmitting an ACK/NACK signal is usedas a retransmission control signal, and a NACK is transmitted throughthe PHICH to request retransmission. When retransmission is notrequested, an ACK is transmitted through the PHICH. Retransmission canbe indicated only through a PHICH in the non-adaptive retransmission,which in turn, provides an advantage of a low overhead for the controlsignal transmitted through DL needed in order to indicateretransmission.

Moreover, in the adaptive retransmission, an ACK is transmitted througha PHICH while retransmission and a resource for retransmission areindicated with a UL grant for indicating resource allocationinformation. A UL grant has a bit called an NDI (New Data Indicator),and this bit takes a binary value of 0 or 1. The terminal compares theNDI of a received current UL grant with the NDI of the last UL grant inthe same retransmission process (HARQ (Hybrid ARQ) process). If there isa change in the NDI, the terminal judges that new data is assigned. Ifthere is no change in the NDI, the terminal judges that retransmissiondata is assigned. In the adaptive retransmission, the resource amountand MCS (Modulation and Coding Scheme) can be changed according to arequired SINR (Signal-to-Interference and Noise power Ratio) of theretransmission signal, which therefore provides an advantage ofimproving the frequency use efficiency.

A UL grant includes CRC (Cyclic Redundancy Check) and therefore thereliability of the received signal is high in comparison with a PHICH.Thus, when receiving a PHICH and a UL grant, the terminal follows anindication of the UL grant.

FIG. 1 illustrates a UL retransmission control procedure in a terminal.In FIG. 1, at Step (hereinafter abbreviated as “ST”) 11, whether a ULgrant exists is determined. If a UL grant exists (YES), the processshifts to ST12. If a UL grant does not exist (NO), the process shifts toST15.

In ST12, the NDI of the current UL grant is compared to the NDI of thelast UL grant in the same retransmission process to determine whetherthe NDI changes. If the NDI changes (YES), the process shifts to ST13.If the NDI does not change (NO), the process shifts to ST14

In ST13, new data is transmitted to the base station. In ST14,retransmission data is adaptively retransmitted to the base station.

In ST15, whether a PHICH is NACK is determined. If the PHICH is NACK(YES), the process shifts to ST16. If the PHICH is not NACK (NO), theprocess shifts to ST17.

In ST16, retransmission data is non-adaptively retransmitted to the basestation. In ST17, the process is shift to suspending, i.e.,retransmission control is suspended.

Here, mapping of a PHICH will be explained. First, in encoding of aPHICH, an ACK/NACK (1 bit) is subject to three times repetition. Thenumber of PHICHs is one of {⅙, ½, 1, 2} times as many as the number ofRBs and is indicated through a PBCH (Physical Broadcast Channel). Bycode multiplex with SF=4 and IQ multiplex, eight PHICHs can betransmitted with 3 REGs (=12 REs). Eight PHICHs placed at 3 REGs arecalled a PHICH group.

Mapping of a PHICH is dependent on a cell ID. Therefore, an interferencecontrol with other cells is difficult, and a PHICH interferes withPDCCHs of other cells and/or a CRS (Cell-specific Reference Signal). All3 REGs included in a PHICH may be mapped to OFDM symbol #0, or the REGsmay be respectively mapped to OFDM symbols #0, #1, and #2 as illustratedin FIG. 2. Information representing which mapping is used is indicatedusing broadcast information.

In LTE and LTE-A, each RB (Resource Block) consists of 12subcarriers×0.5 msec, and the unit of combination of two RBs in the timedomain is called an RB pair. Therefore, an RB pair consists of 12subcarriers×1 msec. When representing a block of 12 subcarriers in thefrequency domain, the RB pair may only be called an RB. Moreover, theunit of one subcarrier x one OFDM symbol is called one RE (ResourceElement). Moreover, one REG (Resource Element Group) includes four REs.

Under the premise described above, NPL 1 proposes that, since thereception quality of a PHICH is inadequate in a terminal at a cell edge,adaptive retransmission without using a PHICH is set in a higher layer.When only adaptive retransmission is supported without using a PHICH,retransmission is controlled only with a UL grant. Therefore, raisingthe aggregation level of the UL grant can improve the reception qualityof a terminal at a cell edge.

Moreover, in NPL 2, the operation without using a PHICH is applied in abackhaul between a base station and a relay station. This is because arelay station transmits a control signal region to a terminal and thusdoes not receive a PHICH for the purpose of preventing interference dueto coupling loop.

In this way, whether or not to use a PHICH is determined in units ofterminals or systems in NPL 1 and NPL 2.

CITATION LIST Non-Patent Literature NPL 1

R1-074811 Semi-static Configuration of Non-adaptive and Adaptive HARQ inE-UTRA Downlink

NPL 2

3GPP TS 36.216 V10.1.0 Physical layer for relaying operation

BRIEF SUMMARY Technical Problem

However, subframes having various characteristics are introduced inLTE-A, and whether use of a PHICH is suitable is dependent on asubframe. Therefore, when a PHICH is used with a subframe unsuitable foruse of a PHICH, the reception quality of PHICH may decrease, forexample, due to interference with a PHICH from another cell, orinterference may be given to another cell. As a result, there arises aproblem of a decrease in system throughput.

It is an object of the present invention to provide a communicationapparatus and a retransmission control method that avoid a decrease insystem throughput due to use of a PHICH.

Solution to Problem

A communication apparatus according to an aspect of the presentinvention includes: a physical HARQ indicator channel (PHICH) receivingsection that, when receiving a PHICH including an ACK or NACKtransmitted from another communication apparatus, does not receive thePHICH in units of communication in which the communication apparatusmonitors an enhanced physical downlink control channel (E-PDCCH), butreceives the PHICH in units of communication in which the communicationapparatus does not monitor an E-PDCCH; and a retransmission controlsection that controls whether to perform retransmission to the othercommunication apparatus, based on the received PHICH and assignmentcontrol information.

A communication apparatus according to an aspect of the presentinvention includes: a physical HARQ indicator channel (PHICH) receivingsection that, when receiving a PHICH including an ACK or NACKtransmitted from another communication apparatus, does not receive aPHICH corresponding to a physical uplink shared channel (PUSCH) assignedby assignment control information on an enhanced physical downlinkcontrol channel (E-PDCCH), but receives a PHICH corresponding to a PUSCHassigned by assignment control information on a PDCCH; and aretransmission control section that controls whether to performretransmission to the other communication apparatus, based on thereceived PHICH and assignment control information.

A retransmission control method according to an aspect of the presentinvention includes: when a physical HARQ indicator channel (PHICH)including an ACK or NACK transmitted from a first communicationapparatus is received, not receiving by a second communicationapparatus, the PHICH in units of communication in which the secondcommunication apparatus monitors an enhanced physical downlink controlchannel (E-PDCCH), but receiving by the second communication apparatus,the PHICH in units of communication in which the second communicationapparatus does not monitor an E-PDCCH; and controlling whether toperform retransmission to the first communication apparatus, based onthe received PHICH and assignment control information.

A retransmission control method according to an aspect of the presentinvention includes: when a physical HARQ indicator channel (PHICH)including an ACK or NACK transmitted from a first communicationapparatus is received, not receiving by a second communicationapparatus, a PHICH corresponding to a physical uplink shared channel(PUSCH) assigned to the second communication apparatus by assignmentcontrol information on an enhanced physical downlink control channel(E-PDCCH), but receiving by the second communication apparatus, a PHICHcorresponding to a PUSCH assigned to the second communication apparatusby assignment control information on a PDCCH; and controlling whether toperform retransmission to the first communication apparatus, based onthe received PHICH and assignment control information.

Advantageous Effects of Invention

The present invention makes it possible to avoid a decrease in systemthroughput due to use of a PHICH.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a UL retransmission controlprocedure according to the relate art;

FIG. 2 illustrates mapping of PHICHs;

FIG. 3 is a block diagram illustrating the main configuration of a radiocommunication apparatus according to the present invention;

FIG. 4 is a block diagram illustrating the configuration of a basestation according to Embodiment 1 of the present invention;

FIG. 5 is a block diagram illustrating the configuration of a terminalaccording to Embodiment 1 of the present invention;

FIG. 6 illustrates an explanation of a retransmission control accordingto Embodiment 1 of the present invention; and

FIG. 7 illustrates an explanation of a retransmission control accordingto Embodiment 2 of the present invention.

DETAILED DESCRIPTION

The inventors of the present invention have accomplished the presentinvention with a focus on the following points (1) to (4) as a subframeincapable of receiving a PHICH or unsuitable for transmitting a PHICHand also with a focus on the following point (5) as a subframe suitablefor a PHICH. Hereinafter, (1) to (5) will be explained.

(1) Subframe Transmitting Control Signal Via E-PDCCH Using Beamforming

A terminal at a cell edge can barely receive an E-PDCCH because ofbeamforming, but cannot receive a PHICH because power boosting cannot beused for a PHICH.

(2) Extension Carrier (Carrier Without PDCCH Region)

In an extension carrier, if a PHICH region is not set in a case where acontrol signal is transmitted using an E-PDCCH, OFDM symbol #0 can beused as an E-PDCCH. Meanwhile, it is easier to use an extension carrierwhen data is assigned from OFDM symbol #0.

(3) Interference Control

Interference control for E-PDCCH can be performed in the frequencydomain. However, the position of a resource allocated to a PHICH isdependent on the cell ID, so that interference control is difficult. Forthis reason, the reception quality of a PHICH decreases in a subframewith large interference from another cell. Moreover, no PHICH should betransmitted in a subframe that should not cause interference with othercells because a PHICH can be the source of interference.

Moreover, the cycle (8 msec cycle in FDD) of UL HARQ process may notmatch the cycle of interference control. When an MBSFN (MBMS SingleFrequency Network) subframe is set to an ABS (Almost Blank Subframe) inFDD, the MBSFN subframe is limited to #0, 4, 5, and 9 subframes.Accordingly, the cycle of UL HARQ process does not match the cycle ofinterference control. Therefore, even if a UL grant transmitted in asubframe without interference, a PHICH may be transmitted in a subframeafter 8 msec which causes large interference.

(4) CoMP (Cooperative Multi-Point) (Reception of Control Signals fromMultiple Cells)

In particular, in a scenario called CoMP scenario 4 for transmittingsignals from multiple transmission points having the same cell ID,PHICHs are subject to channel estimation with CRSs and are to betransmitted from all transmission points. Therefore, the number ofPHICHs may be insufficient. Moreover, PHICHs are transmitted frommultiple transmission points, so that there is a problem of an increasein interference with other cells.

(5) Control Signals in MU-MIMO (Multi User MIMO)

In a cell using MU-MIMO, signals for a plurality of terminals can bemultiplexed and transmitted via a data channel. On the other hand,control signals are transmitted individually to the terminals, so thatthere is a problem of an increase in the amount of control signal. Insuch a case, a PHICH enables transmission by multiplexing signals of aplurality of terminals with a small amount of signal and is thereforeeffective in reducing the overhead for control signals.

A main configuration of radio communication apparatus 50 according tothe present invention will be explained with reference to FIG. 3. InFIG. 3, PHICH receiving section 51 receives a PHICH including an ACK orNACK transmitted from a communication apparatus of the other party andoutputs the received PHICH to retransmission control section 52.However, PHICH receiving section 51 does not receives PHICH in asubframe in which radio communication apparatus 50 monitors an E-PDCCH.

Retransmission control section 52 controls whether to performretransmission to the communication apparatus of the other party on thebasis of the PHICH outputted from PHICH receiving section 51 andassignment control information.

Hereinafter, the embodiments according to the present invention will beexplained in detail with reference to the accompanying drawings.

(Embodiment 1)

A communication system according to Embodiment 1 of the presentinvention includes a transmitting apparatus and a receiving apparatus.In particular, Embodiment 1 of the present invention will be explainedwith the transmitting apparatus as base station 100 and the receivingapparatus as terminal 200. This communication system is an LTE-A system,for example. Additionally, base station 100 is, for example, an LTE-Abase station, and terminal 200 is, for example, an LTE-A terminal.

FIG. 4 is a block diagram illustrating a configuration of base station100 according to Embodiment 1 of the present invention. In this drawing,error detection section 101 receives as input a received data signaloutputted from error correction decoding section 111 described below anddetects whether the reception data includes an error using a CRC or thelike. The detection result is outputted to assignment informationgenerating section 102.

When a data signal to be transmitted through the DL exists, assignmentinformation generating section 102 determines a resource for assigningthe data signal and generates DL assignment as assignment information.Moreover, when a data signal to be assigned to the UL exists, assignmentinformation generating section 102 determines a resource for assigningthe data signal and generates a UL grant as assignment information. Thegenerated assignment information is outputted as information to betransmitted through an E-PDCCH or PDCCH to signal assignment section106. Moreover, the UL grant is outputted to radio receiving section 109in order to receive UL data.

Moreover, assignment information generating section 102 judges whetherto retransmit a signal based on the determination result outputted fromerror detection section 101. When retransmission is not necessary, orwhen no signal needs to be adaptively retransmitted, assignmentinformation generating section 102 instructs PHICH generating section103 to generate an ACK. When non-adaptively retransmitting a signal,assignment information generating section 102 instructs PHICH generatingsection 103 to generate a NACK.

PHICH generating section 103 generates an ACK or NACK according to aninstruction from assignment information generating section 102. However,in the present embodiment, since whether to transmit a PHICH changes foreach subframe, the generated ACK or NACK is outputted to signalassignment section 106 only in a subframe for transmitting a PHICH.Here, in a subframe in which the terminal monitors an E-PDCCH, PHICHgenerating section 103 stops generation of a PHICH and outputs no PHICH.

Error correction coding section 104 receives as an input a transmissiondata signal, performs error correction coding on the receivedtransmission data signal and outputs the signal to modulation section105.

Modulation section 105 modulates the signal outputted from errorcorrection coding section 104 and outputs the modulation signal tosignal assignment section 106.

Signal assignment section 106 maps the modulation signal outputted frommodulation section 105 to a resource on the basis of DL assignmentoutputted from assignment information generating section 102. Moreover,signal assignment section 106 assigns the DL assignment and UL grantoutputted from assignment information generating section 102 to anE-PDCCH region. Furthermore, when a PHICH is outputted from PHICHgenerating section 103, signal assignment section 106 also assigns thePHICH.

The transmission signal is thus generated by assigning the transmissiondata signal, assignment information and PHICH to predeterminedresources. The generated transmission signal is outputted to radiotransmission section 107.

Radio transmission section 107 performs a predetermined radiotransmission process such as up-conversion on the transmission signaloutputted from signal assignment section 106 and transmits the processedsignal via an antenna 108.

Radio receiving section 109 receives a signal transmitted from theterminal via antenna 108 and performs a predetermined radio receptionprocess such as down-conversion on the received signal. Then, radioreceiving section 109 demultiplexes the signal transmitted from theterminal on the basis of UL grant outputted from assignment informationgenerating section 102 and outputs the demultiplexed signal todemodulation section 110.

Demodulation section 110 performs a demodulation process on the signaloutputted from radio receiving section 109 and outputs the resultantdemodulation signal to error correction decoding section 111.

Error correction decoding section 111 decodes the demodulation signaloutputted from demodulation section 110 and acquires a received datasignal. The acquired received data signal is outputted to errordetection section 101.

FIG. 5 is a block diagram illustrating a configuration of terminal 200according to Embodiment 1 of the present invention. In this drawing,radio receiving section 202 receives a signal transmitted from the basestation via antenna 201, performs a predetermined radio receptionprocess such as down-conversion on the signal and outputs the signalresulting from the radio reception process to signal demultiplexingsection 203.

Signal demultiplexing section 203 extracts a control signal and thesignal of the PHICH region from the signal outputted from radioreceiving section 202 and outputs the extracted control signal and thesignal of the PHICH region to control signal receiving section 207 andPHICH receiving section 206, respectively.

Moreover, signal demultiplexing section 203 extracts from the receivedsignal a signal (that is, DL data signal) assigned to a data resourceindicated by mapping resource pattern information outputted from controlsignal receiving section 207 described below and outputs the extractedsignal to demodulation section 204.

Demodulation section 204 demodulates the signal outputted from signaldemultiplexing section 203, and outputs the demodulation signal to errorcorrection decoding section 205.

Error correction decoding section 205 decodes the demodulation signaloutputted from demodulation section 204 and outputs the acquiredreceived data signal.

PHICH receiving section 206 determines whether the signal of the PHICHregion is ACK or NACK. In the present embodiment, since whether toreceive a PHICH changes for each subframe, the determination isperformed only in a subframe for receiving a PHICH. The ACK or NACKbased on the determination result is outputted to control signalreceiving section 207. In addition, in a subframe for receiving noPHICH, an ACK is outputted to control signal receiving section 207.Here, in a subframe in which terminal 200 monitors an E-PDCCH, terminal200 receives no PHICH.

Control signal receiving section 207 performs blind decoding on thecontrol signal extracted by signal demultiplexing section 203 anddetects a control signal intended for the terminal, i.e., DL assignmentor UL grant. Control signal receiving section 207 outputs the detectedDL assignment to signal demultiplexing section 203 and outputs thedetected UL grant to signal assignment section 210.

Control signal receiving section 207 also serves as a retransmissioncontrol section and outputs a signal indicating non-adaptiveretransmission to signal assignment section 210, when the signaloutputted from PHICH receiving section 206 is NACK and also when no ULgrant is detected. Moreover, when the signal outputted from PHICHreceiving section 206 is ACK and also when no UL grant is detected,control signal receiving section 207 does not output a signal indicatingthe assignment to signal assignment section 210.

Error correction coding section 208 receives as an input transmissiondata signal, performs error correction coding on the receivedtransmission data signal and outputs the signals to modulation section209.

Modulation section 209 modulates the signal outputted from errorcorrection coding section 208 and outputs the modulation signal tosignal assignment section 210.

If a UL grant is outputted from control signal receiving section 207,signal assignment section 210 compares the NDI of the current UL grantwith the NDI of the last UL grant in the same retransmission process.Then, if the NDI changes, signal assignment section 210 judges that newdata is assigned and maps the modulation signal of the new dataoutputted from modulation section 209 according to the UL grant. On theother hand, if the NDI does not change, signal assignment section 210judges that retransmission data is assigned and maps the modulationsignal of the retransmission data outputted from modulation section 209according to the UL grant. Furthermore, if retransmission instructionsignals are inputted, signal assignment section 210 maps the modulationsignal of the retransmission data outputted from modulation section 209according to the last UL grant in the same retransmission process. Themapped signal is outputted to radio transmission section 211 as atransmission signal.

Radio transmission section 211 performs a predetermined radiotransmission process such as up-conversion on the transmission signaloutputted from signal assignment section 210 and transmits the processedsignal via an antenna 201.

In this way, in PHICH generating section 103 of the base station andPHICH receiving section 206 of the terminal described above, it isdefined that no PHICH is used in a subframe in which an E-PDCCH ismonitored. Therefore, the terminal performs adaptive retransmission inresponse to an instruction of retransmission by the UL grant in asubframe in which an E-PDCCH is monitored, without receiving a PHICH. AnE-PDCCH is used also for the purpose of interference control. In thiscase, a subframe using an E-PDCCH is a subframe receiving interferenceor causing interference.

Accordingly, it is made possible to prevent using a PHICH difficult tobe received in a subframe receiving interference from another cell orcausing interference to another cell by a PHICH in a subframe causinginterference. In this way, when interference control is performed inunits of cells, Embodiment 1 is effective in particular.

In addition, in a subframe in which an E-PDCCH is monitored, a resourcesubject to interference control in the frequency domain can be used totransmit a UL grant. In the present embodiment, since no PHICH is usedin a subframe in which an E-PDCCH is monitored, no PHICH is used even ifa UL grant is received via a PDCCH or an E-PDCCH, when an E-PDCCH and aPDCCH are used together.

Next, a retransmission control between base station 100 illustrated inFIG. 4 and terminals 200 illustrated in FIG. 5 will specifically beexplained taking FDD as an example with reference to FIG. 6.

In FDD, the UL HARQ process cycle is eight subframes. For example, in DLsubframes #2 to 4, 7, 9, 12 and 13, the terminal is configured tomonitor an E-PDCCH. A PDCCH is also used concomitantly in a subframe inwhich an E-PDCCH is monitored. In DL subframes #2 to 4, 7, 9, 12 and 13,the terminal receives no PHICH. However, the base station may transmit aPHICH for a conventional terminal.

The terminal receives a PHICH in a subframe in which no E-PDCCH ismonitored, and therefore receives a PHICH in DL subframes #0, 1, 5, 6,8, 10 and 11.

In FIG. 6, a new PUSCH of UL subframe #4 is assigned by the UL grant inthe PDCCH region of DL subframe #0. UL subframe #4 corresponds to ULHARQ process #4. Similarly, a new PUSCH of UL subframe #5 is assigned bythe UL grant of the PDCCH region of DL subframe #1. UL subframe #5corresponds to UL HARQ process #5.

The retransmission instruction for the PUSCH transmitted in UL subframe#4 is issued in DL subframe #8, which corresponds to the fourth subframefrom UL subframe #4. Additionally, the retransmission instruction forthe PUSCH transmitted in UL subframe #5 is issued in DL subframe #9,which corresponds to the fourth subframe from UL subframe #5.

Since DL subframe #8 is not a subframe in which an E-PDCCH is monitored,the terminal receives a PHICH. Therefore, if receiving PHICH=NACK andnot detecting a UL grant on a PDCCH or E-PDCCH, the terminal transmits aretransmission signal in UL subframe #12 (UL HARQ process #4) using thesame resource (by non-adaptive retransmission).

On the other hand, since DL subframe #9 is a subframe in which anE-PDCCH is monitored, the terminal does not receive a PHICH. Therefore,the transmission of a retransmission signal is instructed through aPDCCH or E-PDCCH. Here, the retransmission instruction is received withthe UL grant including NDI=0 (no change in NDI from the last assignment)through the E-PDCCH. Therefore, in UL subframe #13, the retransmissionsignal is transmitted using the indicated resource (by adaptiveretransmission).

In this way, according to Embodiment 1, it is defined that no PHICH isused in a subframe in which the terminal monitors an E-PDCCH. Therefore,the UL grant indicates the retransmission instruction in this case, sothat it is made possible to prevent using a PHICH difficult to bereceived in a subframe receiving interference from another cell orcausing interference to another cell by a PHICH in a subframe causinginterference. As a result, it is made possible to avoid a decrease inthroughput due to use of a PHICH.

(Embodiment 2)

The configuration of a base station according to Embodiment 2 of thepresent invention is the same as the configuration illustrated in FIG. 4of Embodiment 1 except for the operation of PHICH generating section103. Therefore, only PHICH generating section 103 will be explained withreference to FIG. 4.

PHICH generating section 103 generates an ACK or NACK signal accordingto an instruction from assignment information generating section 102.However, in the present embodiment, since whether to transmit a PHICHchanges for each subframe, the generated ACK or NACK is outputted tosignal assignment section 106 only in a subframe in which a PHICH istransmitted. Here, generation of a PHICH serving as a retransmissioninstruction for the PUSCH to which a resource is assigned by the ULgrant through an E-PDCCH is stopped, and no PHICH is outputted in thiscase.

The configuration of a terminal according to Embodiment 2 of the presentinvention is the same as the configuration illustrated in FIG. 5 ofEmbodiment 1 except for the operation of PHICH receiving section 206.Therefore, only PHICH receiving section 206 will be explained withreference to FIG. 5.

PHICH receiving section 206 determines whether the signal in the PHICHregion is ACK or NACK. In the present embodiment, since whether toreceive a PHICH changes for each subframe, the determination isperformed only in a subframe in which a PHICH is received. The ACK orNACK based on the determination result is outputted to control signalreceiving section 207. In addition, in a subframe in which no PHICH isreceived, an ACK is outputted to control signal receiving section 207.Here, the terminal does not receive any PHICH serving as aretransmission instruction for the PUSCH to which a resource is assignedby the UL grant of an E-PDCCH.

In this way, in the present embodiment, it is defined that, when aresource assigned by the UL grant through an E-PDCCH, no PHICH is usedfor a retransmission instruction for the PUSCH assigned by an E-PDCCH asdescribed above. Therefore, the terminal does not receive a PHICHcorresponding to a PUSCH assigned by an E-PDCCH. On the other hand, theterminal receives a PHICH corresponding to a PUSCH assigned by a PDCCH.

Thereby, the use of a PHICH can be switched dynamically as compared withEmbodiment 1. More specifically, in a subframe in which both a PDCCH andan E-PDCCH are monitored, when a PHICH is used for the nextretransmission instruction, a PUSCH is assigned by a PDCCH. On the otherhand, when no PHICH is used for the next retransmission instruction, aPUSCH is assigned by an E-PDCCH. Thereby, the use of a PHICH can beswitched dynamically.

Moreover, when power is increased using an E-PDCCH for a terminal withlow reception quality at a cell edge, or when transmission is performedwhile directing a beam to enhance the reception quality for such aterminal, the terminal with low reception quality may be not able toreceive a PDCCH and a PHICH due to the low reception quality.

In the present embodiment, if an E-PDCCH is transmitted to such aterminal in the first transmission, the terminal dose not receive aPHICH with low reception quality even if a subframe in which theretransmission instruction is issued is not a subframe in which anE-PDCCH is monitored. This enables suspending until retransmission canbe instructed next through an E-PDCCH. Moreover, when a PDCCH with ahigh aggregation level is available, a retransmission instruction can bereceived through a PDCCH. In this way, when power control is performedin units of terminals, Embodiment 2 is effective particularly.

Next, a retransmission control between the base stations and theterminal according to Embodiment 2 of the present invention willspecifically be explained taking FDD as an example with reference toFIG. 7.

For example, in DL subframes #1 to 4, 7, 8, and 11 to 13, an E-PDCCH isset so as to be monitored. A PDCCH is also used concomitantly in asubframe in which an E-PDCCH is monitored. The terminal does not receivea PHICH corresponding to a PUSCH assigned through an E-PDCCH.

In FIG. 7, a new PUSCH of UL subframe #4 is assigned by the UL grant inthe PDCCH region of DL subframe #0. UL subframe #4 corresponds to ULHARQ process #4. Similarly, a new PUSCH of UL subframe #5 is assigned bythe UL grant in the E-PDCCH region of DL subframe #1. UL subframe #5corresponds to UL HARQ process #5.

The retransmission instruction for the PUSCH transmitted in UL subframe#4 is issued in DL subframe #8, which corresponds to the fourth subframefrom UL subframe #4. Therefore, the terminal receives a PHICH in DLsubframe #8. Additionally, the retransmission instruction for a PUSCHtransmitted in UL subframe #5 is issued in DL subframe #9, whichcorresponds to the fourth subframe from UL subframe #5. Therefore, theterminal does not receive PHICH in DL subframe #9.

If receiving PHICH=NACK in DL subframe #8 and not detecting a UL grantin a PDCCH or E-PDCCH, the terminal transmits a retransmission signal inUL subframe #12 (UL HARQ process #4) using the same resource (bynon-adaptive retransmission). On the other hand, in DL subframe #9, thetransmission of the retransmission signal is instructed through a PDCCH.Here, due to no retransmission instruction through a PDCCH, suspendingis enabled, and the terminal does not transmit any retransmission signalin UL subframe #13.

In this way, according to Embodiment 2, it is defined that a PHICH isnot used for a retransmission instruction of a PUSCH assigned by anE-PDCCH when a resource is assigned by the UL grant through an E-PDCCH.The retransmission instruction corresponding to this case is issued bythe UL grant. Thereby, in a subframe in which both a PDCCH and anE-PDCCH are monitored, a PDCCH is used for the assignment when a PHICHis used for the next retransmission instruction, and an E-PDCCH is usedfor the assignment when a PHICH is not used for the next retransmissioninstruction. Thereby, according to whether the operation of a PHICH issuitable, the use of a PHICH can be switched dynamically.

(Embodiment 3)

Embodiment 3 of the present invention will be explained in the casewhere the presence or absence of a PHICH is controlled in cooperationwith a measurement set for measuring the DL channel quality.

In LTE-A, a situation where the interference level differs in each DLsubframe occurs. In particular, such a situation occurs in the casewhere an ABS (Almost Blank Subframe) is configured. An ABS is a subframein which the base station reduces transmission power (including the casewhere no transmission is performed in the subframe). For example, asubframe which is configured as an ABS by cell A is a subframe withsmall interference for neighboring cell B. The information regarding theABS configuration is information exchanged between base stations and isnot indicated to the terminal.

For this reason, since the terminal does not know the ABS configuration,such a scheme has been introduced that the terminal selects only asubframe in a certain interference state (assuming that an ABS or anon-ABS is configured in another cell), measures the channel quality andreports the measurement result to the base station. The base stationindicates which subframes can be assumed to have the same interferencestates as one another, to the terminals, with a bit string called ameasurement set. The terminal measures the channel quality of a limitedsubframe with the measurement set and reports the measurement result tothe base station. In this way, measurement of the channel quality ofonly subframes in a certain interference state improves the accuracy ofchannel quality estimation. In other words, this reduces the differencebetween the channel quality which has been reported to the base stationby the terminal and the channel quality when the terminal receives asignal through a subframe in which the base station has transmitted asignal in reality. However, if the channel quality of a certain subframeis measured and reported from among subframes having differentinterference states, the difference between the reported channel qualityand the channel quality of a subframe in which signals assigned actuallymay increase.

Moreover, measurement patterns include CSI (Channel State Information)measurement and mobility measurement. CSI measurement is measurementused for scheduling in the base station. More specifically, thismeasurement is used for measuring, for example, the average of the wholeband, the channel quality of a division band resource obtained bydividing a band, called a sub-band, a rank number receivable during MIMOtransmission, and precoding with excellent properties.

When subframes in which CSI measurement is performed are limited, thepatterns of subframes (referred to as subframe sets) are set to twosubsets. For example, subset 1 is set as subframes #0, 1, 5, 6, 7, 9, .. . (set up to subframe #39, i.e., in a 40-subframe cycle, and subset 2is set as subframes #2, 3, 8, . . . (set up to subframe #39). There maybe a subframe not included in either one of the subsets.

On the other hand, mobility measurement indicates the channel quality ofa connected cell and a neighboring cell and is used in order to for thebase station to judge whether the terminal is moved to the neighboringcell. Reference signal received power (RSRP) averaged for a longerperiod than CSI and reference signal received quality (RSRQ) are used,for example. When subframes for mobility measurement are limited,patterns of subframes are set as one subset for a connected cell and asone subset for a neighboring cell. This is because the subframereceiving larger interference differs depending on the cell. Thismeasurement set is also indicated in a 40-subframe cycle.

The configuration of the base station according to Embodiment 3 of thepresent invention is the same as the configuration illustrated in FIG. 4of Embodiment 1, and the configuration of the terminal according toEmbodiment 3 of the present invention is the same as the configurationillustrated in FIG. 5 of Embodiment 1, except that the operations ofPHICH generating section 103 of base station 100 and PHICH receivingsection 206 of terminal 200 differ depending on the following cases(A-1) and (A-2).

(A-1) PHICH generating section 103 of base station 100 and PHICHreceiving section 206 of terminal 200 limit the use of a PHICH incooperation with a CSI measurement set. In this case, it is associatedthat no PHICH is used in the use of one of the two subsets as a CSImeasurement set and that a PHICH is used in the use of the other subset.Which subset to associate with the use of a PHICH may be indicated bynew signaling or may beforehand be determined in the system.

(A-2) When PHICH generating section 103 of base station 100 and PHICHreceiving section 206 of terminal 200 operate with a mobilitymeasurement set, they are associated with the setting in which no PHICHis used in the use of one of a subset used for a connection cell and asubset configured for a neighboring cell and a PHICH is used in the useof the other subset. Which subset to associate with the use of a PHICHmay be indicated by additional signaling. Moreover, whether a PHICH isused for a subframe reported so as to be measured using a measurementset or for a subframe indicated so as not to be measured may be reportedby additional signaling.

In this way, according to Embodiment 3, association with the setting inwhich a PHICH is used in one of two subsets in a CSI measurement set ora mobility measurement set and no PHICH is used in the other subset.Thereby, the use of a PHICH can be linked with an interference state,and no PHICH can be assigned only to a terminal at a cell edge.

In addition, although an ABS subframe is not indicated to a terminal atpresent, the presence or absence of a PHICH may be determined inconjunction with an ABS subframe if an ABS subframe is indicated to aterminal.

Moreover, if a subframe for indicating an interference situation of eachsubframe is newly added in addition to an ABS subframe, the presence orabsence of a PHICH may be determined in conjunction with the indication.

(Embodiment 4)

Embodiment 4 of the present invention will be explained in the case ofsetting whether a PHICH is used in conjunction with setting of an MBSFNsubframe.

An MBSFN subframe is a subframe defined for transmitting MBMS(Multimedia Broadcast Multicast Service) data. Although data cannot beassigned to a terminal demodulating data using a CRS since there is noCRS in the data region, a DMRS (Demodulation Reference signal) can beused in LTE-A to assign data to a terminal. Whether a PHICH is used inan MBSFN subframe or in a subframe that is not an MBSFN subframe(non-MBSFN subframe) may separately be indicated by additional signalingor may beforehand be determined in the system.

The configuration of the base station according to Embodiment 4 of thepresent invention is the same as the configuration illustrated in FIG. 4of Embodiment 1, and the configuration of the terminal according toEmbodiment 4 of the present invention is the same as the configurationillustrated in FIG. 5 of Embodiment 1, except that the operations ofPHICH generating section 103 of base station 100 and PHICH receivingsection 206 of terminal 200 differ depending on the following cases(B-1) and (B-2).

(B-1) First, the case will be explained where PHICH generating section103 of base station 100 and PHICH receiving section 206 of terminal 200use a PHICH in an MBSFN subframe and do not use a PHICH in a non-MBSFNsubframe.

An MBSFN subframe has no CRS and is therefore suitable for MU-MIMO formultiplexing signals of multiple terminals using DMRSs. Therefore, whenthere are many terminals performing MU-MIMO, the availability of controlsignal regions (PDCCH and E-PDCCH) becomes tight. Consequently, in anMBSFN subframe, the resource amount used for control signals can bereduced by using a PHICH. It is assumed that a probability of MU-MIMOmultiplexing is low for a terminal with such low reception quality thatreception of a PHICH is difficult. As a result, a terminal subject toMU-MIMO is assumed to have high reception quality of a PHICH and cantherefore receive a PHICH.

(B-2) Next, the case will be explained where PHICH generating section103 of base station 100 and PHICH receiving section 206 of terminal 200use no PHICH in an MBSFN subframe and use a PHICH in a non-MBSFNsubframe.

An MBSFN subframe has no CRS unlike the scenario of MU-MIMO of (B-1)described above and may therefore be used as an interference controlsubframe. As a result, an MBSFN subframe serves as a subframe receivinginterference or a subframe causing interference. Using no PHICH in anMBSFN subframe can prevent using a PHICH difficult to be received in asubframe receiving interference from another cell or causinginterference to another cell by a PHICH in a subframe causinginterference. In particular, the largest number of OFDM symbols in aPDCCH region in an MBSFN subframe is small in comparison with that in anon-MBSFN subframe, therefore leading to a low effect of power boostingof a PHICH. Therefore, it is difficult to receive a PHICH in a subframereceiving interference.

In this way, Embodiment 4 sets whether to use a PHICH in conjunctionwith setting of an MBSFN subframe and thereby achieve the following. (1)A shortage of control signal regions due to terminals subject to MU-MIMOmultiplexing can be lightened when a PHICH is used in an MBSFN subframeand is not used in a non-MBSFN subframe. (2) It is possible to preventusing a PHICH difficult to be received in a subframe receivinginterference and causing interference to another cell by a PHICH in asubframe giving interference can be prevented when a PHICH is not usedin an MBSFN subframe and is used in a non-MBSFN subframe.

(Embodiment 5)

Embodiment 5 of the present invention will be explained in the casewhere whether to use a PHICH is determined on the basis of N_(g) ∈ {⅙,½, 1, 2} that is a coefficient for determining the number of PHICHs.

N_(g) ∈ {⅙, ½, 1, 2} is a value indicated by signaling in a higher layerand is used for following Equation 1 determining the number of PHICHgroups. As a result, an increase in the value of N_(g) increases thenumber of PHICHs which can be transmitted in one subframe.

$\begin{matrix}{\left( {{Equation}\mspace{14mu} 1} \right)\mspace{619mu}} & \; \\{N_{PHICH}^{group} = \left\{ \begin{matrix}\left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil & {{for}\mspace{14mu} {normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}} \\{2 \cdot \left\lceil {N_{g}\left( {N_{RB}^{DL}/8} \right)} \right\rceil} & {{for}\mspace{14mu} {extended}\mspace{14mu} {cyclic}\mspace{11mu} {prefix}}\end{matrix} \right.} & \lbrack 1\rbrack\end{matrix}$

The configuration of the base station according to Embodiment 5 of thepresent invention is the same as the configuration illustrated in FIG. 4of Embodiment 1, and the configuration of the terminal according toEmbodiment 5 of the present invention is the same as the configurationillustrated in FIG. 5 of Embodiment 1, except that the operations ofPHICH generating section 103 of base station 100 and PHICH receivingsection 206 of terminal 200 involve the presence of a PHICH in the caseof N_(g)≧P and the absence of a PHICH in the case of N_(g)<P. However,the present embodiment uses P∈ {½, 1, 2} as an example. That is, thevalue of N_(g) can be increased to use a PHICH in the case where thereare many terminals subject to MU-MIMO multiplexing, and the value ofN_(g) can be decreased to use no PHICH in the case of a small number ofPHICHs which can be transmitted in one subframe. The value of P isbeforehand determined in the system, or may differ in each terminal.

In this way, Embodiment 5 determines whether to use a PHICH inconjunction with coefficient N_(g) determining the number of PHICHs.Thereby, the value of N_(g) can be increased to use a PHICH togetherwith an E-PDCCH in the case where there are many terminals subject toMU-MIMO multiplexing, and the value of N_(g) can be decreased to use noPHICH in the case of a small number of PHICHs which can be transmittedin one subframe.

In addition, the present embodiment is more effective when used incombination with the other embodiments. For example, in combination withEmbodiment 1, although it is defined that a PHICH is not used in asubframe in which an E-PDCCH is monitored, a PHICH may be used even in asubframe in which an E-PDCCH is monitored when N_(g)≧P. In this way,whether each of the above-described embodiments is applied for each cellcan be set according to the value of Ng.

(Embodiment 6)

Embodiment 6 of the present invention will be explained in the casewhere the presence or absence of a PHICH is indicated using a bit in theUL grant.

Although a UL grant is transmitted in DCI (DL Control Information)format #0 or 4, the number of formats may increase in the future.

The base station indicates the presence or absence of a PHICH usinginformation on the DCI format forming the UL grant. The terminal judgeswhether there is a PHICH for the PUSCH assigned by the UL grant, on thebasis of bits in the UL grant. If the presence or absence of a PHICH canbe indicated by the UL grant in this manner, the presence or absence ofa PHICH can be switched dynamically.

The bit in the UL grant is provided by adding a new dedicated bit orusing a bit for other information. When a bit for other information isused, it is possible to use bits for indicating a cyclic shift of aDMRS. The bits consist of three bits, and a cyclic shift value isdefined as “0, 2, 3, 4, 6, 8, 9, and 10” and is indicated from the basestation to the terminal using three bits.

In setting of these cyclic shift values, it is assumed that indicating acertain shift value represents the absence of a PHICH, the other shiftvalues represent the presence of a PHICH. In particular, the cyclicshift values of 3 and 9 are consecutive to other shift values and areunlikely to be used during MU-MIMO. Therefore, if the cyclic shiftvalues of 3 and 9 are set to represent the absence of a PHICH while theother cyclic shift values set to represent the presence of a PHICH, thissetting provides an advantage of allowing easier association with theoperations of MU-MIMO.

The embodiments have been explained thus far.

(C-1) Extension of the Present Invention to Carrier Aggregation

In the case of non-cross carrier scheduling, cell-specific configurationsuch as using a PHICH in Pcell (Primary cell) and not using a PHICH inScell (Secondary cell), is possible. Meanwhile, in the case of crosscarrier scheduling, the configuration in the Pcell is used since a PHICHis available only in Pcell.

(C-2) Extension of the Present Invention to Extension Carrier

An extension carrier is a carrier in which signals to be mapped in aregion determined in units of cells, such as a PDCCH, PHICH, PCFICH,CRS, and the like are not mapped. Possible schemes of assigning a dataregion using an extension carrier include applying cross carrierscheduling to transmit a control signal from a carrier that is not anextension carrier, and using an E-PDCCH region in which a resource isassigned to each UE.

The case of applying cross carrier scheduling follows the configurationof the carrier that transmits a control signal. Alternatively, it ispossible to define that no PHICH is used for an extension carrier. Whenan E-PDCCH is used for assignment, no PHICH is used in an extensioncarrier.

(C-3) Extension to SPS (Semi Persistent Scheduling)

Schemes of assigning DL data and UL data include dynamic scheduling andsemi persistent scheduling (SPS). Dynamic scheduling indicates aresource for first transmission for each subframe using DL assignment orUL grant. In SPS, first transmission of HARQ (Hybrid ARQ) isperiodically transmitted with the same resource until indication ofcompletion is acquired after the resource is indicated by DL assignmentor UL grant.

Here, the term “first transmission” refers to the initial transmissionof HARQ and corresponds to the operation of retransmission using theresource indicated by the retransmission instruction or the sameresource according to a retransmission instruction when a PHICH is usedby UL even in the case of both dynamic scheduling and SPS duringretransmission. When assignment to small packets for, for example, audiocommunication, is indicated using a control signal each time, theoverhead for control signals increases relative to the amount of thepackets. Therefore, SPS is suitable for the case where small packetsfor, for example, audio communication occur continuously. Therefore, useof a PHICH is an effective scenario in this case.

Consequently, a PHICH is used for assignment using SPS as exceptionhandling. Thereby, when the overhead for control signals is large incomparison with the data volume, retransmission can be indicated by aPHICH, and the resources for control signals can be used effectively.

In addition, the operations of each of the embodiment described aboveare implemented only in specific terminals and are not implemented inLTE terminals or rel. 10 LTE-A terminals, thereby maintaining backwardcompatibility. Moreover, these operations may be applied only toterminals supporting an E-PDCCH or CoMP. Those terminals can bedistinguished using the category, rel. number, or the like of theterminals. Moreover, these operations may not be applied to terminalshaving a specified TM (Transmission mode) supporting UL MIMO.

If these operations are not applied to terminals (configured with DCIformat 4) supporting UL MIMO transmission, a PHICH can positively beused during MU-MIMO.

In the embodiments described above, the present invention is configuredwith hardware by way of example, but the invention may also be providedby software in concert with hardware.

In addition, the functional blocks used in the description of theembodiments are typically implemented as LSI devices, which areintegrated circuits. The functional blocks may be formed as individualchips, or a part or all of the functional blocks may be integrated intoa single chip. The term “LSI” is used herein, but the terms “IC,”“system LSI,” “super LSI” or “ultra LSI” may be used as well dependingon the level of integration.

In addition, the circuit integration is not limited to LSI and may beachieved by dedicated circuitry or a general-purpose processor otherthan LSI. After fabrication of LSI, a field programmable gate array(FPGA), which is programmable, or a reconfigurable processor that allowsreconfiguration of connections and settings of circuit cells in LSI maybe used.

Should a circuit integration technology replacing LSI appear as a resultof advancements in semiconductor technology or other technologiesderived from the technology, the functional blocks could be integratedusing such a technology. Another possibility is the application ofbiotechnology and/or the like.

The disclosure of the specification, drawings, and abstract included inJapanese Patent Application No. 2011-176864, filed on Aug. 12, 2011, isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The communication apparatus and retransmission control method accordingto the present invention can be applied to, for example, a mobilecommunication system represented by LTE-A.

REFERENCE SIGNS LIST

-   100 Base station-   101 Error detection section-   102 Assignment information generating section-   103 PHICH generating section-   104, 208 Error correction coding section-   105, 209 Modulation section-   106, 210 Signal assignment section-   107, 211 Radio transmission section-   108, 201 Antenna-   109, 202 Radio receiving section-   110, 204 Demodulation section-   111, 205 Error correction decoding section-   200 Terminal-   203 Signal demultiplexing section-   206 PHICH receiving section-   207 Control signal receiving section

1. A communication apparatus comprising: a physical HARQ indicatorchannel (PHICH) receiver that, when receiving a PHICH including an ACKor NACK transmitted from another communication apparatus, does notreceive the PHICH in units of communication in which the communicationapparatus monitors a non-cross carrier scheduled secondly cell, butreceives the PHICH in units of communication in which the communicationapparatus monitors a primary cell; and a retransmission controller that,in operation, controls whether to perform retransmission to the othercommunication apparatus, based on the received PHICH and assignmentcontrol information.
 2. A communication apparatus comprising: a physicalHARQ indicator channel (PHICH) receiver that, when receiving a PHICHincluding an ACK or NACK transmitted from another communicationapparatus, does not receive a PHICH corresponding to a physical uplinkshared channel (PUSCH) assigned by assignment control information on anon-cross carrier scheduled secondly cell, but receives a PHICHcorresponding to a PUSCH assigned by assignment control information on aprimary cell; and a retransmission controller that, in operation,controls whether to perform retransmission to the other communicationapparatus, based on the received PHICH and assignment controlinformation.
 3. The communication apparatus according to claim 1,wherein the PHICH receiver, in operation, receives a PHICH in one of twosubsets in a channel state information (CSI) measurement set or amobility measurement set and does not receive a PHICH in the other oneof the two subsets.
 4. The communication apparatus according to claim 1,wherein the PHICH receiver, in operation, receives a PHICH in an MBMSsingle frequency network (MBSFN) subframe, but does not receive a PHICHin a non-MBSFN subframe.
 5. The communication apparatus according toclaim 1, wherein the PHICH receiver, in operation, does not receive aPHICH in an MBMS single frequency network (MBSFN) subframe, but receivesa PHICH in a non-MBSFN subframe.
 6. The communication apparatusaccording to claim 1, wherein the PHICH receiver, in operation,determines whether to receive a PHICH, based on the number of PHICHs. 7.A retransmission control method comprising: when a physical HARQindicator channel (PHICH) including an ACK or NACK transmitted from afirst communication apparatus is received, not receiving by a secondcommunication apparatus, the PHICH in units of communication in whichthe second communication apparatus monitors an enhanced physicaldownlink control channel (E-PDCCH), but receiving by the secondcommunication apparatus, the PHICH in units of communication in whichthe second communication apparatus does not monitor an E-PDCCH; andcontrolling whether to perform retransmission to the first communicationapparatus, based on the received PHICH and assignment controlinformation.
 8. A retransmission control method comprising: when aphysical HARQ indicator channel (PHICH) including an ACK or NACKtransmitted from a first communication apparatus is received, notreceiving by a second communication apparatus, a PHICH corresponding toa physical uplink shared channel (PUSCH) assigned to the secondcommunication apparatus by assignment control information on a non-crosscarrier scheduled secondly cell, but receiving by the secondcommunication apparatus, a PHICH corresponding to a PUSCH assigned tothe second communication apparatus by assignment control information ona primary cell; and controlling whether to perform retransmission to thefirst communication apparatus, based on the received PHICH andassignment control information.